As of version 3.2 on iOS and 2.9.1 on Mac, photos will no longer be optimized but will be full resolution. This is for photos added after updating to these versions.
Every photo imported into Day One is converted to JPEG format and resized to a maximum resolution of 2100 pixels, Premium users have the benefit of an increased resolution at 3200 pixels.
The aspect ratio is maintained. We resize photos for more efficient sync and storage. At this size the average photo is about 700KB to 1.5MB.
When you take a photo using Day One on your iPhone or iPad, by default the original, full-resolution image is not saved to your Camera Roll. You can change this behavior in the Day One Settings > Advanced panel.